The present invention relates generally to a superconductive magnet used to generate a high magnetic field as part of a magnetic resonance imaging (MRI) diagnostic system, and more particularly to such a magnet having an open design and a magnetic field of high uniformity.
MRI systems employing superconductive or other type magnets are used in various fields such as medical diagnostics. Known superconductive magnets include liquid-helium cooled and cryocooler-cooled superconductive magnets. Typically, for a cryocooler-cooled magnet, the superconductive coil assembly includes a superconductive main coil surrounded by a thermal shield surrounded by a vacuum enclosure. A cryocooler coldhead is externally mounted to the vacuum enclosure, has its first stage in thermal contact with the thermal shield, and has its second stage in thermal contact with the superconductive main coil. Nb--Ti superconductive coils typically operate at a temperature of generally 4 Kelvin, and Nb--Sn superconductive coils typically operate at a temperature of generally 10 Kelvin.
Known superconductive magnet designs include closed magnets and open magnets. Closed magnets typically have a single, tubular-shaped superconductive coil assembly having a bore. The superconductive coil assembly includes several radially-aligned and longitudinally spaced-apart superconductive main coils each carrying a large, identical electric current in the same direction. The superconductive main coils are thus designed to create a magnetic field of high uniformity within a spherical imaging volume centered within the magnet's bore where the object to be imaged is placed. Although the magnet is so designed to have a highly uniform magnetic field within the imaging volume, manufacturing tolerances in the magnet and magnetic field disturbances caused by the environment at the field site of the magnet usually require that the magnet be corrected at the field site for such minor irregularities in the magnetic field. Typically, the magnet is shimmed at the field site by using pieces of iron, or, for Nb--Ti superconductive magnets cooled by liquid helium, by using numerous Nb--Ti superconductive correction coils. The correction coils are placed within the superconductive coil assembly radially near and radially inward of the main coils. Each correction coil carries a different, but low, electric current in any required direction including a direction opposite to the direction of the electric current carried in the main coils. Shielding coils may also be used within the superconductive coil assembly to prevent the high magnetic field created by and surrounding the main coils from adversely interacting with electronic equipment in the vicinity of the magnet. Such shielding coils carry electric current of generally equal amperage, but in an opposite direction, to the electric current carried in the main coils and are positioned radially outward of the main coils.
Open magnets typically employ two spaced-apart superconductive coil assemblies with the space between the assemblies allowing for access by medical personnel for surgery or other medical procedures during MRI imaging. The patient may be positioned in that space or also in the bore of the toroidal-shaped coil assemblies. The open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design. Before Applicants' invention, known superconductive open magnets existed only in the literature. However, the literature is silent on how such magnets can be designed to have a magnetic field of high uniformity within the imaging volume when the creation of the open space between the superconductive coil assemblies grossly distorts the magnetic field creating a magnetic field of low uniformity within the imaging volume. Such magnetic field distortion is well beyond that which can be overcome by using known magnet shimming technology.
What is needed is an open MRI magnet designed to have a highly uniform magnetic field within its imaging volume despite the gross magnetic field distortions created by the open space between the magnet's superconductive coil assemblies.